By connecting two pnpn-devices in parallel, a sensitive optical receiver-transmitter (transceiver) can be obtained, with uncritical operation with respect to temperature and bias voltages. A common resistance is connected in series with the pair to allow only one of the pnpn-devices to switch ON.
Such a system is described in Japanese Patent JP-A-3-235926. Synchronous operation is then performed, i.e., light is first input to one or both of the devices in the pair and then voltage is applied to the pair. The pnpn-device which collected the highest number of photons will turn ON and emit light. The other element will remain OFF. Information can thus be received and transmitted. A measure for the optical sensitivity is the required energy of the optical input pulses to obtain reliable detection.
In a previous pnpn-device described in International Patent Application No. PCT/BE93/00074 submitted by the present inventors entitled `Fast Electrical Complete Turn-off Optical Device`, we proposed to change the layer structure in such a way that a single pnpn-device and a differential pair of pnpn-devices can be reset quickly with a negative anode to cathode voltage pulse. See also, M. Kuijk, P. Heremans, G. Borghs and R. Vounckx: `Depleted Double-Interojunction Optical Thyristor`, Appl. Phys. Lett. vol. 64, 2073 (1994).
Complete depletion of the central base layers, which allows a high bit-rate operation without loss of optical sensitivity can be achieved. This has resulted so far in a demonstration of cascadable operation at 50 MHz and 7.4 femtojoule optical sensitivity, which is described in B. Knupfer, M. Kuijk, P. Heremans, R. Vounckx and G. Borghs: `Cascadable Differential pnpn Optoelectronic Switch Operating at 50 Mbit/sec with Ultra-High Optical Input Sensitivity` accepted by Electr. Lttrs (Reference EU/94/45575).
Until now, the use of such pairs has been limited to optical-to-optical transceiver systems. Optical digital information synchronized with the electrical pulsed-power-supply can be received, amplified and retransmitted. Although the information undergoes an optical to electrical and an electrical to optical transformation, it is not obvious to read out or to induce the state of the differential pair electrically.
The straight-forward way to induce the state of the pair, and also to read it out, would be by contacting one of the middle layers of the pnpn-devices. Such a contact is usually called the "gate" of a pnpn device. However, the connected gate and its connected metal conductors will act as an antenna, and disturb the very sensitive operation of the pair. Moreover, if the sensitive operation with complete depletion, such as described in above-mentioned patent application `Fast Electrical Complete Turn-Off Optical Device`, is to be used, the gates of the pnpn-devices cannot be connected, since the associated base layers become completely depleted after each reset operation.
An alternative for achieving optical interconnects is to use conventional light-emitters and receivers. The light emitters would then be either lasers or LED's requiring high-current driving capability of the Si-circuitry. On the side of the light receivers, several amplifier stages are required to obtain a digital output, wasting useful Si area. See, M. Ingels, G. V. d. Plas, J. Crols and M. Steyaert: `A CMOS 18 THz.OMEGA. 240 Mb/s Transimpedance Amplifier and 155 Mb/s LED Driver for Low-Cost Optical Fiber Links`, IEEE J. of Solid-state Circuits, Vol. 29, 1552-1559 (1995).